Charged particle assessment tool, inspection method

ABSTRACT

A charged particle assessment tool includes: an objective lens configured to project a plurality of charged particle beams onto a sample, the objective lens having a sample-facing surface defining a plurality of beam apertures through which respective ones of the charged particle beams are emitted toward the sample; and a plurality of capture electrodes adjacent respective ones of the beam apertures and configured to capture charged particles emitted from the sample.

This application claims the benefit of priority to European patentapplication no. 20198201.4, filed Sep. 24, 2020, European patentapplication no. 20184160.8, filed Jul. 6, 2020, and of European patentapplication no. 20150394.3, filed Jan. 6, 2020, each of which isincorporated herein its entirety by reference.

FIELD

The embodiments provided herein generally relate to charged-particleassessment tools and inspection methods, and particularly tocharged-particle assessment tools and inspection methods that usemultiple sub-beams of charged particles.

BACKGROUND

When manufacturing semiconductor integrated circuit (IC) chips or otherdevices, undesired pattern defects, as a consequence of, for example,optical effects and incidental particles, inevitably occur on asubstrate (e.g., a wafer) or a patterning device (e.g., a mask) duringthe fabrication processes, thereby reducing the yield. Monitoring theextent of undesired pattern defects is therefore a significant processin device manufacture. More generally, the inspection and/or measurementof a surface of a substrate, or other object/material, is a significantprocess during and/or after its manufacture.

Pattern inspection tools with a charged particle beam have been used toinspect objects, for example to detect pattern defects. These toolstypically use electron microscopy techniques, such as a scanningelectron microscope (SEM). In a SEM, a primary electron beam ofelectrons at a relatively high energy is targeted with a finaldeceleration step in order to land on a sample at a relatively lowlanding energy. The beam of electrons is focused as a probing spot onthe sample. The interactions between the material structure at theprobing spot and the landing electrons from the beam of electrons causeelectrons to be emitted from the surface, such as secondary electrons,backscattered electrons or Auger electrons. The generated secondaryelectrons may be emitted from the material structure of the sample. Byscanning the primary electron beam as the probing spot over the samplesurface, secondary electrons can be emitted across the surface of thesample. By collecting these emitted secondary electrons from the samplesurface, a pattern inspection tool may obtain an image representingcharacteristics of the material structure of the surface of the sample.

SUMMARY

There is a general need to improve the throughput and othercharacteristics of charged particle inspection tools.

It is an object of the present disclosure to provide, for example,embodiments that support improvement of throughput or othercharacteristics of charged-particle assessment tools.

According to an aspect, there is provided a charged particle assessmenttool comprising:

an objective lens configured to project a plurality of charged particlebeams onto a sample, the objective lens defining a plurality of beamapertures through which respective ones of the charged particle beamscan propagate toward the sample; and

a plurality of sensor units adjacent respective ones of the beamapertures and configured to capture charged particles emitted from thesample.

According to an aspect, there is provided a method of manufacturing anassessment tool, the method comprising:

forming a plurality of sensor units on, and a plurality of apertures in,a substrate; and

attaching the substrate to an objective lens configured to project aplurality of charged particle beams onto a sample, so that the chargedparticle beams can be emitted through the apertures.

According to an aspect, there is provided an inspection methodcomprising:

emitting a plurality of charged-particle beams through a plurality ofbeam apertures to a sample; and

capturing charged particles emitted by the sample in response to thecharged-particle beams using a plurality of sensor units providedadjacent respective ones of the beam apertures.

According to an aspect, there is provided a multi-beam electron-opticalsystem comprising a last electron-optical element in a multi-beam pathof the multi-beam electron optical system, the last electron-opticalelement comprising:

a multi-manipulator array in which each array element is configured tomanipulate at least one electron beam in the multi-beam path; and

a detector configured and orientated to detect electrons emitted from asample positioned in the multi-beam beam path, wherein the detectorcomprises a plurality of sensor units integrated into themulti-manipulator array and at least one sensor unit associated witheach array element.

According to an aspect, there is provided a last electron-opticalelement for a multi-charged beam projection system configured to projecta plurality of charged particle beams onto a sample, the lastelectron-optical element comprising:

an objective lens having a sample-facing surface defining a plurality ofbeam apertures through which respective ones of the charged particlebeams can propagate toward the sample; and

a plurality of sensor units adjacent respective ones of the beamapertures and configured to capture charged particles emitted from thesample.

Advantages of embodiments of the present invention will become apparentfrom the following description taken in conjunction with theaccompanying drawings wherein are set forth, by way of illustration andexample, certain embodiments of the present invention.

BRIEF DESCRIPTION OF FIGURES

The above and other aspects of the present disclosure will become moreapparent from the description of exemplary embodiments, taken inconjunction with the accompanying drawings.

FIG. 1 is a schematic diagram illustrating an exemplary charged particlebeam inspection apparatus.

FIG. 2 is a schematic diagram illustrating an exemplary multi-beamapparatus that is part of the exemplary charged particle beam inspectionapparatus of FIG. 1 .

FIG. 3 is a schematic diagram of exemplary multi-beam apparatusillustrating an exemplary configuration of source conversion unit of theexemplary charged particle beam inspection apparatus of FIG. 1 .

FIG. 4 is a schematic cross-sectional view of an objective lens of aninspection apparatus according to an embodiment.

FIG. 5 is a bottom view of the objective lens of FIG. 4 .

FIG. 6 is a bottom view of a modification of the objective lens of FIG.4 .

FIG. 7 is an enlarged schematic cross-sectional view of a detectorincorporated in the objective lens of FIG. 4 .

FIG. 8 is a schematic diagram of a theoretical trans impedanceamplifier.

FIG. 9 is a schematic diagram of a trans impedance amplifier indicatingthe effect of thermal noise.

FIG. 10 is a schematic diagram illustrating an exemplary multi-beamapparatus that may be part of the exemplary charged particle beaminspection apparatus of FIG. 1 .

FIG. 11 is a schematic diagram illustrating an exemplary multi-beamapparatus according to an embodiment.

FIG. 12 is a schematic diagram of an exemplary multi-beam apparatusaccording to an embodiment.

FIG. 13 is a schematic cross-sectional view of an objective lens of aninspection apparatus according to an embodiment.

FIG. 14 is a bottom view of a detector unit incorporated in theobjective lens of FIG. 13 .

FIG. 15 is a schematic diagram illustrating an exemplary multi-beamapparatus according to an embodiment.

FIG. 16 is an enlarged schematic cross-sectional view of a detector thatcan be incorporated in the objective lens of the apparatus of FIG. 15with the detector in different positions.

FIG. 17 is an enlarged schematic cross-sectional view of a detector thatcan be used in the objective lens of the apparatus of FIG. 15 .

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses and methods consistent with aspects related tothe invention as recited in the appended claims.

The enhanced computing power of electronic devices, which reduces thephysical size of the devices, can be accomplished by significantlyincreasing the packing density of circuit components such astransistors, capacitors, diodes, etc. on a device such as an IC chip.This has been enabled by increased resolution enabling yet smallerstructures to be made. For example, an IC chip of a smart phone, whichis the size of a thumbnail and available in, or earlier than, 2019, mayinclude over 2 billion transistors, the size of each transistor beingless than 1/1000th of a human hair. Thus, it is not surprising thatsemiconductor IC manufacturing is a complex and time-consuming process,with hundreds of individual steps. Errors in even one step have thepotential to dramatically affect the functioning of the final product.Just one “killer defect” can cause device failure. The goal of themanufacturing process is to improve the overall yield of the process.For example, to obtain a 75% yield for a 50-step process (where a stepcan indicate the number of layers formed on a wafer), each individualstep must have a yield greater than 99.4%. If an individual step has ayield of 95%, the overall process yield would be as low as 7%.

While high process yield is desirable in a device manufacturingfacility, maintaining a high substrate (i.e. wafer) throughput, definedas the number of substrates processed per hour, is also significant.High process yield and high substrate throughput can be impacted by thepresence of a defect. This is especially if operator intervention isrequired for reviewing defects. Thus, high throughput detection andidentification of micro and nano-scale defects by inspection tools (suchas a Scanning Electron Microscope (SEM)) is used to maintain high yieldand low cost.

A SEM comprises a scanning device and a detector apparatus. The scanningdevice comprises an illumination apparatus that comprises an electronsource, for generating primary electrons, and a projection apparatus forscanning a sample, such as a substrate, with one or more focused beamsof primary electrons. Together at least the illumination apparatus, orillumination system, and the projection apparatus, or projection system,may be referred to together as the electron-optical system or apparatus.The primary electrons interact with the sample and generate secondaryelectrons. The detection apparatus captures the secondary electrons fromthe sample as the sample is scanned so that the SEM can create an imageof the scanned area of the sample. For high throughput inspection, someof the inspection apparatuses use multiple focused beams, i.e. amulti-beam, of primary electrons. The component beams of the multi-beammay be referred to as sub-beams or beamlets. A multi-beam can scandifferent parts of a sample simultaneously. A multi-beam inspectionapparatus can therefore inspect a sample at a much higher speed than asingle-beam inspection apparatus.

In a multi-beam inspection apparatus, the paths of some of the primaryelectron beams are displaced away from the central axis, i.e. amid-point of the primary electron optical axis, of the scanning device.To help ensure all the electron beams arrive at the sample surface withsubstantially the same angle of incidence, sub-beam paths with a greaterradial distance from the central axis need to be manipulated to movethrough a greater angle than the sub-beam paths with paths closer to thecentral axis. This stronger manipulation may cause aberrations whichresult in blurry and out-of-focus images of the sample substrate. Inparticular, for sub-beam paths that are not on the central axis, theaberrations in the sub-beams may increase with the radial displacementfrom the central axis. Such aberrations may remain associated with thesecondary electrons when they are detected. Such aberrations thereforedegrade the quality of images that are created during inspection.

An implementation of a multi-beam inspection apparatus is describedbelow.

The figures are schematic. Relative dimensions of components in drawingsare therefore exaggerated for clarity. Within the following descriptionof drawings the same or like reference numbers refer to the same or likecomponents or entities, and only the differences with respect to theindividual embodiments are described. While the description and drawingsare directed to an electron-optical apparatus, it is appreciated thatthe embodiments are not used to limit the present disclosure to specificcharged particles. References to electrons throughout the presentdocument may therefore be more generally be considered to be referencesto charged particles, with the charged particles not necessarily beingelectrons.

Reference is now made to FIG. 1 , which is a schematic diagramillustrating an exemplary charged particle beam inspection apparatus100. The charged particle beam inspection apparatus 100 of FIG. 1includes a main chamber 10, a load lock chamber 20, an electron beamtool 40, an equipment front end module (EFEM) 30 and a controller 50.Electron beam tool 40 is located within main chamber 10.

EFEM 30 includes a first loading port 30 a and a second loading port 30b. EFEM 30 may include one or more additional loading ports. Firstloading port 30 a and second loading port 30 b may, for example, receivesubstrate front opening unified pods (FOUPs) that contain one or moresubstrates (e.g., semiconductor substrates or substrates made of othermaterial(s)) or samples to be inspected (substrates, wafers and samplesare collectively referred to as “samples” hereafter). One or more robotarms (not shown) in EFEM 30 transport the samples to load lock chamber20.

Load lock chamber 20 is used to remove the gas around a sample. Thiscreates a vacuum that is a local gas pressure lower than the pressure inthe surrounding environment. The load lock chamber 20 may be connectedto a load lock vacuum pump system (not shown), which removes gasparticles in the load lock chamber 20. The operation of the load lockvacuum pump system enables the load lock chamber to reach a firstpressure below the atmospheric pressure. After reaching the firstpressure, one or more robot arms (not shown) transport the sample fromload lock chamber 20 to main chamber 10. Main chamber 10 is connected toa main chamber vacuum pump system (not shown). The main chamber vacuumpump system removes gas particles in main chamber 10 so that thepressure in around the sample reaches a second pressure lower than thefirst pressure. After reaching the second pressure, the sample istransported to the electron beam tool by which it may be inspected. Anelectron beam tool 40 may comprise a multi-beam electron-opticalapparatus.

Controller 50 is electronically connected to electron beam tool 40.Controller 50 may be a processor (such as a computer) configured tocontrol the charged particle beam inspection apparatus 100. Controller50 may also include a processing circuitry configured to execute varioussignal and image processing functions. While controller 50 is shown inFIG. 1 as being outside of the structure that includes main chamber 10,load lock chamber 20, and EFEM 30, controller 50 may be part of thestructure. The controller 50 may be located in one of the componentelements of the charged particle beam inspection apparatus or it can bedistributed over at least two of the component elements. While thepresent disclosure provides examples of main chamber 10 housing anelectron beam inspection tool, it should be noted that aspects of thedisclosure in their broadest sense are not limited to a chamber housingan electron beam inspection tool. Rather, it is appreciated that theforegoing principles may also be applied to other tools and otherarrangements of apparatus, that operate under the second pressure.

Reference is now made to FIG. 2 , which is a schematic diagramillustrating an exemplary electron beam tool 40 including a multi-beaminspection tool that is part of the exemplary charged particle beaminspection apparatus 100 of FIG. 1 . Multi-beam electron beam tool 40(also referred to herein as apparatus 40) comprises an electron source201, a gun aperture plate 271, a condenser lens 210, a source conversionunit 220, a primary projection apparatus 230, a motorized stage 209, anda sample holder 207. The electron source 201, gun aperture plate 271,condenser lens 210, and source conversion unit 220 are the components ofan illumination apparatus comprised by the multi-beam electron beam tool40. The sample holder 207 is supported by motorized stage 209 so as tohold a sample 208 (e.g., a substrate or a mask) for inspection.Multi-beam electron beam tool 40 may further comprise a secondaryprojection apparatus 250 and an associated electron detection device240. Primary projection apparatus 230 may comprise an objective lens231, e.g. a unitary lens operating on the whole beam. An objective lensmay be the last electron-optical element in the path of the multi-beamor in the electron-optical system; so, the objective lens may bereferred to as a type of last electron-optical element. Electrondetection device 240 may comprise a plurality of detection elements 241,242, and 243. A beam separator 233 and a deflection scanning unit 232may be positioned inside primary projection apparatus 230.

The components that are used to generate a primary beam may be alignedwith a primary electron-optical axis 204 of the apparatus 40. Thesecomponents can include: the electron source 201, gun aperture plate 271,condenser lens 210, source conversion unit 220, beam separator 233,deflection scanning unit 232, and primary projection apparatus 230.Secondary projection apparatus 250 and its associated electron detectiondevice 240 may be aligned with a secondary electron-optical axis 251 ofapparatus 40.

The primary electron-optical axis 204 is comprised by theelectron-optical axis of the part of electron beam tool 40 that is theillumination apparatus. The secondary electron-optical axis 251 is theelectron-optical axis of the part of electron beam tool 40 that is adetection apparatus. The primary electron-optical axis 204 may also bereferred to herein as the primary optical axis (to aid ease ofreference) or charged particle optical axis. The secondaryelectron-optical axis 251 may also be referred to herein as thesecondary optical axis or the secondary charged particle optical axis.

Electron source 201 may comprise a cathode (not shown) and an extractoror anode (not shown). During operation, electron source 201 isconfigured to emit electrons as primary electrons from the cathode. Theprimary electrons are extracted or accelerated by the extractor and/orthe anode to form a primary electron beam 202 that forms a primary beamcrossover (virtual or real) 203. Primary electron beam 202 may bevisualized as being emitted from primary beam crossover 203.

In this arrangement a primary electron beam, by the time it reaches thesample, and desirably before it reaches the projection apparatus, is amulti-beam. Such a multi-beam can be generated from the primary electronbeam in a number of different ways. For example, the multi-beam may begenerated by a multi-beam array located before the cross-over, amulti-beam array located in the source conversion unit 220, or amulti-beam array located at any point in between these locations. Amulti-beam array may comprise a plurality of electron beam manipulatingelements arranged in an array across the beam path. Each manipulatingelement may influence the primary electron beam to generate a sub-beam.Thus the multi-beam array interacts with an incident primary beam pathto generate a multi-beam path down-beam of the multi-beam array.

Gun aperture plate 271, in operation, is configured to block offperipheral electrons of primary electron beam 202 to reduce Coulombeffect. The Coulomb effect may enlarge the size of each of probe spots221, 222, and 223 of primary sub-beams 211, 212, 213, and thereforedeteriorate inspection resolution. A gun aperture plate 271 may also bereferred to as a coulomb aperture array.

Condenser lens 210 is configured to focus primary electron beam 202.Condenser lens 210 may be designed to focus primary electron beam 202 tobecome a parallel beam and be normally incident onto source conversionunit 220. Condenser lens 210 may be a movable condenser lens that may beconfigured so that the position of its first principal plane is movable.The movable condenser lens may be configured to be magnetic. Condenserlens 210 may be an anti-rotation condenser lens and/or it may bemovable.

Source conversion unit 220 may comprise an image-forming element array,an aberration compensator array, a beam-limit aperture array, and apre-bending micro-deflector array. The pre-bending micro-deflector arraymay deflect a plurality of primary sub-beams 211, 212, 213 of primaryelectron beam 202 to normally enter the beam-limit aperture array, theimage-forming element array, and an aberration compensator array. Inthis arrangement, the image-forming element array may function as amulti-beam array to generate the plurality of sub-beams in themulti-beam path, i.e. primary sub-beams 211, 212, 213. The image formingarray may comprise a plurality electron beam manipulators such asmicro-deflectors or micro-lenses (or a combination of both) to influencethe plurality of primary sub-beams 211, 212, 213 of primary electronbeam 202 and to form a plurality of parallel images (virtual or real) ofprimary beam crossover 203, one for each of the primary sub-beams 211,212, and 213. The aberration compensator array may comprise a fieldcurvature compensator array (not shown) and an astigmatism compensatorarray (not shown). The field curvature compensator array may comprise aplurality of micro-lenses to compensate field curvature aberrations ofthe primary sub-beams 211, 212, and 213. The astigmatism compensatorarray may comprise a plurality of micro-stigmators to compensateastigmatism aberrations of the primary sub-beams 211, 212, and 213. Thebeam-limit aperture array may be configured to limit cross-sectionalwidths (e.g., diameters) of individual primary sub-beams 211, 212, and213. FIG. 2 shows three primary sub-beams 211, 212, and 213 as anexample, and it should be understood that source conversion unit 220 maybe configured to form any number of primary sub-beams. Controller 50 maybe connected to various parts of charged particle beam inspectionapparatus 100 of FIG. 1 , such as source conversion unit 220, electrondetection device 240, primary projection apparatus 230, or motorizedstage 209. As explained in further detail below, controller 50 mayperform various image and signal processing functions. Controller 50 mayalso generate various control signals to govern operations of thecharged particle beam inspection apparatus, including the chargedparticle multi-beam apparatus.

Condenser lens 210 may further be configured to adjust electric currentsof primary sub-beams 211, 212, 213 down-beam of source conversion unit220 by varying the focusing power of condenser lens 210. Alternatively,or additionally, the electric currents of the primary sub-beams 211,212, 213 may be changed by altering the radial sizes of beam-limitapertures within the beam-limit aperture array corresponding to theindividual primary sub-beams. The electric currents may be changed byboth altering the radial sizes of beam-limit apertures and the focusingpower of condenser lens 210. If the condenser lens is moveable andmagnetic, off-axis sub-beams 212 and 213 may result that illuminatesource conversion unit 220 with rotation angles. The rotation angleschange with the focusing power or the position of the first principalplane of the movable condenser lens. A condenser lens 210 that is ananti-rotation condenser lens may be configured to keep the rotationangles unchanged while the focusing power of condenser lens 210 ischanged. Such a condenser lens 210 that is also movable, may cause therotation angles not to change when the focusing power of the condenserlens 210 and the position of its first principal plane are varied.

Objective lens 231 may be configured to focus sub-beams 211, 212, and213 onto a sample 208 for inspection and may form three probe spots 221,222, and 223 on the surface of sample 208.

Beam separator 233 may be, for example, a Wien filter comprising anelectrostatic deflector generating an electrostatic dipole field and amagnetic dipole field (not shown in FIG. 2 ). In operation, beamseparator 233 may be configured to exert an electrostatic force byelectrostatic dipole field on individual electrons of primary sub-beams211, 212, and 213. The electrostatic force is equal in magnitude butopposite in direction to the magnetic force exerted by magnetic dipolefield of beam separator 233 on the individual electrons. Primarysub-beams 211, 212, and 213 may therefore pass at least substantiallystraight through beam separator 233 with at least substantially zerodeflection angles.

Deflection scanning unit 232, in operation, is configured to deflectprimary sub-beams 211, 212, and 213 to scan probe spots 221, 222, and223 across individual scanning areas in a section of the surface ofsample 208. In response to incidence of primary sub-beams 211, 212, and213 or probe spots 221, 222, and 223 on sample 208, electrons aregenerated from the sample 208 which include secondary electrons andbackscattered electrons. The secondary electrons propagate in threesecondary electron beams 261, 262, and 263. The secondary electron beams261, 262, and 263 typically have secondary electrons (having electronenergy 50 eV) and may also have at least some of the backscatteredelectrons (having electron energy between 50 eV and the landing energyof primary sub-beams 211, 212, and 213). The beam separator 233 isarranged to deflect the path of the secondary electron beams 261, 262,and 263 towards the secondary projection apparatus 250. The secondaryprojection apparatus 250 subsequently focuses the path of secondaryelectron beams 261, 262, and 263 onto a plurality of detection regions241, 242, and 243 of electron detection device 240. The detectionregions may be the separate detection elements 241, 242, and 243 thatare arranged to detect corresponding secondary electron beams 261, 262,and 263. The detection regions generate corresponding signals which aresent to controller 50 or a signal processing system (not shown), e.g. toconstruct images of the corresponding scanned areas of sample 208.

The detection elements 241, 242, and 243 may detect the correspondingsecondary electron beams 261, 262, and 263. On incidence of secondaryelectron beams with the detection elements 241, 242 and 243, theelements may generate corresponding intensity signal outputs (notshown). The outputs may be directed to an image processing system (e.g.,controller 50). Each detection element 241, 242, and 243 may compriseone or more pixels. The intensity signal output of a detection elementmay be a sum of signals generated by all the pixels within the detectionelement.

The controller 50 may comprise image processing system that includes animage acquirer (not shown) and a storage device (not shown). Forexample, the controller may comprise a processor, computer, server,mainframe host, terminals, personal computer, any kind of mobilecomputing device, or the like, or a combination thereof. The imageacquirer may comprise at least part of the processing function of thecontroller. Thus the image acquirer may comprise at least one or moreprocessors. The image acquirer may be communicatively coupled to anelectron detection device 240 of the apparatus 40 permitting signalcommunication, such as an electrical conductor, optical fiber cable,portable storage media, infrared (IR) communication, Bluetooth,internet, wireless network, wireless radio, among others, or acombination thereof. The image acquirer may receive a signal fromelectron detection device 240, may process the data comprised in thesignal and may construct an image therefrom. The image acquirer may thusacquire images of sample 208. The image acquirer may also performvarious post-processing functions, such as generating contours,superimposing indicators on an acquired image, or the like. The imageacquirer may be configured to perform adjustments of brightness,contrast, etc. of acquired images. The storage may be a storage mediumsuch as a hard disk, flash drive, cloud storage, random access memory(RAM), other types of computer readable memory, or the like. The storagemay be coupled with the image acquirer and may be used for savingscanned raw image data as original images, and post-processed images.

The image acquirer may acquire one or more images of a sample based onan imaging signal received from the electron detection device 240. Animaging signal may correspond to a scanning operation for conductingcharged particle imaging. An acquired image may be a single imagecomprising a plurality of imaging areas. The single image may be storedin the storage. The single image may be an original image that may bedivided into a plurality of regions. Each of the regions may compriseone imaging area containing a feature of sample 208. The acquired imagesmay comprise multiple images of a single imaging area of sample 208sampled multiple times over a time period. The multiple images may bestored in the storage. The controller 50 may be configured to performimage processing steps with the multiple images of the same location ofsample 208.

The controller 50 may include measurement circuitry (e.g.,analog-to-digital converters) to obtain a distribution of the detectedsecondary electrons. The electron distribution data, collected during adetection time window, can be used in combination with correspondingscan path data of each of primary sub-beams 211, 212, and 213 incidenton the sample surface, to reconstruct images of the sample structuresunder inspection. The reconstructed images can be used to reveal variousfeatures of the internal or external structures of sample 208. Thereconstructed images can thereby be used to reveal any defects that mayexist in the sample.

The controller 50 may control motorized stage 209 to move sample 208during inspection of sample 208. The controller 50 may enable motorizedstage 209 to move sample 208 in a direction, desirably continuously, forexample at a constant speed, at least during sample inspection. Thecontroller 50 may control movement of the motorized stage 209 so that itchanges the speed of the movement of the sample 208 dependent on variousparameters. For example, the controller may control the stage speed(including its direction) depending on the characteristics of theinspection steps of scanning process.

Although FIG. 2 shows that apparatus 40 uses three primary electronsub-beams, apparatus 40 may use two or more primary electron sub-beams.The present disclosure does not limit the number of primary electronbeams used in apparatus 40.

Reference is now made to FIG. 3 , which is a schematic diagram ofexemplary multi-beam apparatus illustrating an exemplary configurationof source conversion unit of the exemplary charged particle beaminspection apparatus of FIG. 1 . The apparatus 300 may comprise anelectron source 301, a pre-sub-beam-forming aperture array 372, acondenser lens 310 (similar to condenser lens 210 of FIG. 2 ), a sourceconversion unit 320, an objective lens 331 (similar to objective lens231 of FIG. 2 ), and holder to hold a sample 308 (similar to sample 208of FIG. 2 ). The electron source 301, pre-sub-beam-forming aperturearray 372, and condenser lens 310 may be the components of anillumination apparatus comprised by the apparatus 300. The sourceconversion unit 320 and objective lens 331 may be the components of aprojection apparatus comprised by the apparatus 300. The sourceconversion unit 320 may be similar to source conversion unit 220 of FIG.2 in which the image-forming element array of FIG. 2 is image-formingelement array 322, the aberration compensator array of FIG. 2 isaberration compensator array 324, the beam-limit aperture array of FIG.2 is beam-limit aperture array 321, and the pre-bending micro-deflectorarray of FIG. 2 is pre-bending micro-deflector array 323. The electronsource 301, the pre-sub-beam-forming aperture array 372, the condenserlens 310, the source conversion unit 320, and the objective lens 331 arealigned with a primary electron-optical axis 304 of the apparatus. Theelectron source 301 generates a primary-electron beam 302 generallyalong the primary electron-optical axis 304 and with a source crossover(virtual or real) 301S. The pre-sub-beam-forming aperture array 372 cutsthe peripheral electrons of primary electron beam 302 to reduce aconsequential Coulomb effect. The Coulomb effect is a source ofaberration to the sub-beams due to interaction between electrons indifferent sub-beam paths. Primary-electron beam 302 may be trimmed intoa specified number of sub-beams, such as three sub-beams 311, 312 and313, by pre-sub-beam-forming aperture array 372 of apre-sub-beam-forming mechanism. Although three sub-beams and their pathsare referred to in the previous and following description, it should beunderstood that the description is intended to apply an apparatus, tool,or system with any number of sub-beams.

The source conversion unit 320 may include a sub-beam-limit aperturearray 321 with beam-limit apertures configured to limit the sub-beams311, 312, and 313 of the primary electron beam 302. The sourceconversion unit 320 may also include an image-forming element array 322with image-forming micro-deflectors, 322_1, 322_2, and 322_3. There is arespective micro-deflector associated with the path of each sub-beam.The micro-deflectors 322_1, 322_2, and 322_3 are configured to deflectthe paths of the sub-beams 311, 312, and 313 towards theelectron-optical axis 304. The deflected sub-beams 311, 312 and 313 formvirtual images of source crossover 301S. The virtual images areprojected onto the sample 308 by the objective lens 331 and form probespots thereon, which are the three probe spots, 391, 392, and 393. Eachprobe spot corresponds to the location of incidence of a sub-beam pathon the sample surface. The source conversion unit 320 may furthercomprise an aberration compensator array 324 configured to compensateaberrations of each of the sub-beams. The aberrations in each sub-beamare typically present on the probe spots, 391, 392, and 393 that wouldbe formed a sample surface. The aberration compensator array 324 mayinclude a field curvature compensator array (not shown) withmicro-lenses. The field curvature compensator and micro-lenses areconfigured to compensate the sub-beams for field curvature aberrationsevident in the probe spots, 391, 392, and 393. The aberrationcompensator array 324 may include an astigmatism compensator array (notshown) with micro-stigmators. The micro-stigmators are controlled tooperate on the sub-beams to compensate astigmatism aberrations that areotherwise present in the probe spots, 391, 392, and 393.

The source conversion unit 320 may further comprise a pre-bendingmicro-deflector array 323 with pre-bending micro-deflectors 323_1,323_2, and 323_3 to bend the sub-beams 311, 312, and 313, respectively.The pre-bending micro-deflectors 323_1, 323_2, and 323_3 may bend thepath of the sub-beams onto the beamlet-limit aperture array 321. Thesub-beam path of the incident on beamlet-limit aperture array 321 may beorthogonal to the plane of orientation of the beamlet-limit aperturearray 321. The condenser lens 310 may direct the path of the sub-beamsonto the beamlet-limit aperture array 321. The condenser lens 310 mayfocus the three sub-beams 311, 312, and 313 to become parallel beamsalong primary electron-optical axis 304, so that it is essentiallyperpendicularly incident onto source conversion unit 320, which maycorrespond to the beamlet-limit aperture array 321.

The image-forming element array 322, the aberration compensator array324, and the pre-bending micro-deflector array 323 may comprise multiplelayers of sub-beam manipulating devices, some of which may be in theform or arrays, for example: micro-deflectors, micro-lenses, ormicro-stigmators.

In the source conversion unit 320, the sub-beams 311, 312 and 313 of theprimary electron beam 302 are respectively deflected by themicro-deflectors 322_1, 322_2 and 322_3 of image-forming element array322 towards the primary electron-optical axis 304. It should beunderstood that the sub-beam 311 path may already correspond to theelectron-optical axis 304 prior to reaching micro-deflector 322_1,accordingly the sub-beam 311 path may not be deflected bymicro-deflector 322_1.

The objective lens 331 focuses the sub-beams onto the surface of thesample 308, i.e., it projects the three virtual images onto the samplesurface. The three images formed by three sub-beams 311 to 313 on thesample surface form three probe spots 391, 392 and 393 thereon. Thedeflection angles of sub-beams 311 to 313 are adjusted by the objectivelens 311 to reduce the off-axis aberrations of three probe spots391˜393. The three deflected sub-beams consequently pass through orapproach the front focal point of objective lens 331. As depicted theobjective lens 331 is a magnetic lens that focuses all of the sub-beams.In an embodiment, the objective lens is desirably an array ofelectrostatic lenses which may require the multibeam path to be directedby the source conversion unit 320, specifically for example theimage-forming element array 322 featuring micro-deflectors, towards thearray of electrostatic lenses in the objective lens 331. (For example,each beam could be directed towards its own corresponding micro-lens inthe array).

At least some of the above-described components in FIG. 2 and FIG. 3 mayindividually, or in combination with each other, be referred to as amanipulator array, multi-manipulator array, multi-manipulator ormanipulator, because they manipulate one or more beams, or sub-beams, ofcharged particles.

Existing multi-e-beam defect inspection systems have resolution of about2 to 10 nm at a throughput of 10 to 6000 mm² per hour. Such systems havea detector in a secondary column as discussed above. The architecture ofexisting multi-e-beam inspection tools has a detector remote from thesource of electrons emitted from the sample such as back scattered andsecondary electrons, which is not scalable for a many-beam system. It isalso difficult to integrate a secondary column into a tool with an arrayobjective lens, such as an electrostatic lens (which is necessary toaddress Coulomb interactions).

In an embodiment the objective lens referred to in earlier embodimentsis an array objective lens. Typically such a lens arrangement iselectrostatic. Each element in the array is a micro-lens operating on adifferent beam or group of beams in the multi-beam. An electrostaticarray objective lens has at least two plates each with a plurality ofholes or apertures. The position of each hole in a plate corresponds tothe position of a corresponding hole in the other plate. Thecorresponding holes operate in use on the same beam or group of beams inthe multi-beam. A suitable example of a type of lens for each element inthe array is an Einzel lens. The bottom electrode of objective lens is aCMOS chip detector integrated into a multi-beam manipulator array.Integration of a detector array into the objective lens removes the needfor the secondary projection apparatus 250. The CMOS chip is desirablyorientated to face a sample (because of the small distance (e.g. 100 μm)between the substrate and the bottom of the electron-optical system). Inan embodiment, capture electrodes to capture the secondary electronsignals are provided. The capture electrodes can be formed in the metallayer of, for example, a CMOS device. The capture electrode may form thebottom layer of the objective lens. The capture electrode may form thebottom surface in a CMOS chip. The CMOS chip may be a CMOS chipdetector. The CMOS chip may be integrated into the sample facing surfaceof an objective lens assembly. The capture electrodes are examples ofsensor units for detecting secondary electrons. The capture electrodescan be formed in other layers. Power and control signals of the CMOS maybe connected to the CMOS by through-silicon vias. For robustness, thebottom electrode desirably has two elements: the CMOS chip and a passiveSi plate with holes. The plate shields the CMOS from high E-fields.

Sensor units associated with a bottom or sample facing surface of anobjective lens are beneficial because the secondary and/or back-scattedelectrons may be detected before the electrons encounter and becomemanipulated by an electron optical element of the electron-opticalsystem. Beneficially the time taken for detection of such a sampleemanating electron may be reduced, desirably minimized.

In order to maximize the detection efficiency it is desirable to makethe electrode surface as large as possible, so that substantially allthe area of the array objective lens (excepting the apertures) isoccupied by electrodes. Each electrode may have a cross-sectional width(e.g., diameter) substantially equal to the array pitch. The electrodesurfaces may substantially fill the sample-facing surface of the arrayobjective lens. In an embodiment the outer shape of the electrode is acircle, but this can be made a square to maximize the detection area.Also the cross-sectional width (e.g., diameter) of the through-substratehole can be minimized. Typical size of the electron beam is in the orderof 5 to 15 microns.

In an embodiment, a single capture electrode surrounds each aperture.The single capture electrode may have a circular perimeter and/or anouter diameter. The capture electrode may have an area extending betweenthe aperture and the periphery of the capture electrode. As depicted inFIGS. 5 and 6 the capture electrodes (e.g., capture electrodes 405) maybe arranged in rectangular array or a hexagonal array. In an embodiment,a plurality of electrode elements is provided around each aperture. Theplurality of electrode elements may, together, have a circular perimeterand/or a diameter. The plurality of electrode elements may, together,have an area extending between the aperture and the periphery of theplurality of electrode elements. The pluralities of electrode elements(e.g., capture electrodes 405) may be arranged in rectangular array or ahexagonal array. The electrode elements are examples of sensor elements.The electrons captured by the electrode elements surrounding oneaperture may be combined into a single signal or used to generateindependent signals. The electrode elements may be divided radially. Theelectrode elements may form a plurality of concentric annuluses orrings. The electrode elements may be divided radially and/or angularly.The electrode elements may form a plurality of sector-like pieces orsegments. The segments may be of similar angular size and/or similararea. The electrode elements may be divided both radially and angularlyor in any other convenient manner.

However a larger electrode surface leads to a larger parasiticcapacitance, so a lower bandwidth. For this reason it may be desirableto limit the outer cross-sectional width (e.g., diameter) of theelectrode. Especially in case a larger electrode gives only a slightlylarger detection efficiency, but a significantly larger capacitance. Acircular (annular) electrode may provide a good compromise betweencollection efficiency and parasitic capacitance.

A larger outer cross-sectional width (e.g., diameter) of the electrodemay also lead to a larger crosstalk (sensitivity to the signal of aneighboring hole). This can also be a reason to make the electrode outercross-sectional width (e.g., diameter) smaller. Especially in case alarger electrode gives only a slightly larger detection efficiency, buta significantly larger crosstalk.

The back-scattered and/or secondary electron current collected by anelectrode is amplified. The purpose of the amplifier is to enablesufficiently sensitive measurement of the current received or collectedby the sensor unit to be measured and thus the number of back-scatteredand/or secondary electrons. This can be measured by current measurementsor the potential difference over a resistor. Several types of amplifierdesign may be used to amplify back-scattered and/or secondary electroncurrent collected by electrode, such as a transimpedance amplifier(TIA). In such a transimpedance amplifier, the voltage output of the TIAis equal to the TIA resistance (R_(TIA)) multiplied by the measuredcurrent.

The larger R_(TIA), the higher the amplification. However the bandwidthis determined by the RC time, which is equal to R_(TIA) multiplied bythe sum of the capacitances on the entrance side of the TIA.

A finite RC time has a similar effect as a larger electron optics spotsize, so it effectively gives a blur contribution in the deflectiondirection. Given the blur contribution budget of the detector and thedeflection velocity the allowed RC time is determined. Given this RCtime, the entrance capacitance R_(TIA) is determined. Based on theback-scattered and/or secondary electron current and R_(TIA) the signalvoltage is determined.

Noise contributions of the detector should be compared to the shot noiseof the back-scattered and/or secondary electron current. Taking only theshot noise of the primary electron beam into account, the current noiseper sqrt(Hz) due to shot noise is significantly larger than the voltagenoise of a state of the art CMOS amplifier of typically ˜1 nV/sqrt(Hz)as demonstrated below. The rough calculations set out below demonstratethat the proposed electrode is feasible from a noise point of view.

$\begin{matrix}{N_{{PE}\_{def}} = 5000} & (1)\end{matrix}$ $\begin{matrix}{I_{beam} = {1{nA}}} & (2)\end{matrix}$ $\begin{matrix}{d_{def} = {4{nm}}} & (3)\end{matrix}$ $\begin{matrix}{N_{{pix}\_{defect}} = 4} & (4)\end{matrix}$ $\begin{matrix}{{blur}_{rc} = {0.5{nm}}} & (5)\end{matrix}$ $\begin{matrix}{C_{detector} = {{\varepsilon_{0} \cdot \zeta \cdot \frac{0.25 \cdot \pi \cdot \left( {100{µm}} \right)^{2}}{1{µm}}} = {{2.0}86 \times 10^{{- 1}3}F}}} & (6)\end{matrix}$ $\begin{matrix}{I_{{sn}\_{noise}} = {\sqrt{2 \cdot Q_{e} \cdot I_{beam}} = {{1.7}9 \times 10^{{- 1}4}\frac{A}{\sqrt{Hz}}}}} & (7)\end{matrix}$ $\begin{matrix}{t_{defect} = {\frac{N_{{PE}\_{def}} \cdot Q_{e}}{beam} = {{0.8}01{\mu s}}}} & (8)\end{matrix}$ $\begin{matrix}{1_{{scan}\_{defect}}:={{d_{def}\sqrt{N_{{pix}_{defect}}}} = {8{nm}}}} & (9)\end{matrix}$ $\begin{matrix}{v_{scan} = {\frac{1_{{scan}\_{defect}}}{t_{defect}} = {{9.9}86\frac{mm}{s}}}} & (10)\end{matrix}$ $\begin{matrix}{t_{RC} = {\frac{{blur}_{RC}}{v_{scan}} = {5{0.0}69{ns}}}} & (11)\end{matrix}$ $\begin{matrix}{R_{detector} = {\frac{t_{RC}}{C_{detector}} = {2.4 \times 10^{5}\Omega}}} & (12)\end{matrix}$ $\begin{matrix}{V_{sn} = {{I_{{sn}\_{noise}}R_{detector}} = {{4.2}96\frac{nV}{\sqrt{Hz}}}}} & (13)\end{matrix}$ $\begin{matrix}{V_{sn} = {{\frac{\sqrt{2}Q_{e}^{3/2}}{\sqrt{I_{beam}}}\frac{{blur}_{RC}N_{{PE}\_{def}}}{C_{detector}d_{def}\sqrt{N_{{pix}\_{defect}}}}} = {{4.2}96\frac{nV}{\sqrt{Hz}}}}} & (14)\end{matrix}$

The above calculations can be explained as follows. It is assumed thatthe number of primary electrons required to detect a defect is 5000(equation 1), the beam current is 1 nA (equation 2), the diameter of adefect is 4 nm (equation 3) and the number of pixels per defect is 4(equation 4). It is assumed that a blur due to the finite RC time of theamplifier of 0.5 nm is acceptable (equation 5). The capacitance of thedetector can be calculated from the geometry of the arrangement, forexample, as indicated in equation 6 where 3 is the dielectric constantof the insulator underlying the capture electrode, 100 μm is thediameter of the capture electrode and 1 μm is the thickness of theinsulator underneath the capture electrode. The intrinsic shot noise iscalculated as in equation 7. The time to image one defect is calculatedas in equation 8 where Q_(e) is the electron charge. The length of ascan to detect a defect is calculated in equation 9 and the scanningvelocity is calculated in equation 10. The RC time to be achieved iscalculated in equation 11 and therefore the resistance of the detectoris calculated in equation 12 and the resulting voltage noise in equation13. Equation 14 combines the previous equations into a single equationto show dependencies. The typical voltage noise level that can beachieved in CMOS amplifiers is in the order of 1 nV/sqrt(Hz)—which thistypical noise level of a CMOS amplifier. So it is plausible that thenoise is dominated by the fundamental shot noise, and not by the voltagenoise added by the CMOS amplifier. Because of this it is plausible thatthe proposed electrode is feasible from a noise point of view. That is atypical CMOS amplifier noise is sufficiently good to have a noise levelthat is small relative to the shot noise. (Even if it is large relativeto the shot noise, the arrangement could still work, but theeffectiveness in terms of bandwidth or throughput (i.e. speed) could bereduced).

FIG. 8 is a schematic diagram of a theoretical transimpedance amplifier(TIA) in which the voltage output V_(out) is simply the product of themeasured current I_(in) and the feedback resistance R_(f). However, areal TIA has noise, in particular shot noise in the input i_(sn) andthermal noise in the feedback resistor i_(n) as depicted in FIG. 9 . Inmost cases thermal noise dominates. The voltage noise at the outputv_(n) is given by:v _(n)=√{square root over (4·k _(b) ·T·R _(f))}  (15)where k_(b) is the Boltzmann constant. The current noise at the entranceto the TIA is therefore:

$\begin{matrix}{i_{n} = \sqrt{\frac{4 \cdot k_{b} \cdot T}{R_{f}}}} & (16)\end{matrix}$whereas the shot noise is given by:i _(sn)=√{square root over (2·Q _(e) ·I _(in))}  (17)Therefore if the feedback resistance is increased, the thermal noisebecomes lower relative to the shot noise of the input current (i.e. theback-scattered and/or secondary electron current).

It can be shown that the described detector remains practical takingaccount of the effect of shot noise, by assuming, for example, that thenumber of electrons required to detect each defect is increased to10,000; a blur budget of 2 nm is set; and the electrode diameter isreduced to 50 μm. In that case, the capacitance of the electrode becomesabout 0.011 pF requiring a resistance of about 3.6×10⁷Ω, resulting in alevel of thermal noise about 20% greater than the shot noise. Therefore,various different arrangements of the proposed detector are feasible.The capacitance of the electrode can also be controlled by varying thethickness of an adjacent dielectric layer, which may be in the range ofabout 1 to about 5 μm.

An exemplary embodiment is shown in FIG. 4 which illustrates a multibeamobjective lens 401 in schematic cross section. On the output side of theobjective lens 401, the side facing the sample 208, a detector module402 is provided. FIG. 5 is a bottom view of detector module 402 whichcomprises a substrate 404 on which are provided a plurality of captureelectrodes 405 each surrounding a beam aperture 406. Beam apertures 406are large enough not to block any of the primary electron beams. Captureelectrodes 405 can be considered as examples of sensor units whichreceive back-scattered or secondary electrodes and generate a detectionsignal, in this case an electric current. The beam apertures 406 may beformed by etching through substrate 404. In the arrangement shown inFIG. 5 , the beam apertures 406 are shown in a rectangular array. Thebeam apertures 406 can also be differently arranged, e.g. in a hexagonalclose packed array as depicted in FIG. 6 .

FIG. 7 depicts, at a larger scale, a part of the detector module 402 incross section. Capture electrodes 405 form the bottommost, i.e. mostclose to the sample, surface of the detector module 402. Between thecapture electrodes 405 and the main body of the silicon substrate 404, alogic layer 407 is provided. Logic layer 407 may include amplifiers,e.g. transimpedance amplifiers, analog to digital converters, andreadout logic. In an embodiment, there is one amplifier and one analogto digital converter per capture electrode 405. Logic layer 407 andcapture electrodes 405 can be manufactured using a CMOS process with thecapture electrodes 405 forming the final metallization layer.

A wiring layer 408 is provided on the backside of substrate 404 andconnected to the logic layer 407 by through-silicon vias 409. The numberof through-silicon vias 409 need not be the same as the number of beamapertures 406. In particular if the electrode signals are digitized inthe logic layer 407 only a small number of through-silicon vias may berequired to provide a data bus. Wiring layer 408 can include controllines, data lines and power lines. It will be noted that in spite of thebeam apertures 406 there is ample space for all necessary connections.The detection module 402 can also be fabricated using bipolar or othermanufacturing techniques. A printed circuit board and/or one or moreother semiconductor chips may be provided on the backside of detectormodule 402.

FIG. 4 depicts a three electrode objective lens but it will beappreciated that any other form of objective lens, e.g. a two electrodelens, may also be used.

Reference is now made to FIG. 10 , which is a schematic diagramillustrating an exemplary electron beam tool 40 a that may be part ofthe exemplary charged particle beam inspection apparatus 100 of FIG. 1in place of the tool 40 of FIG. 2 . Parts of the apparatus 40 a thathave similar functions as corresponding parts of the apparatus 40 ofFIG. 2 are identified with the same references. A reduced or simplifieddescription of such parts is included below in some cases.

Multi-beam electron beam tool 40 a (also referred to herein as apparatus40 a) comprises an electron source 201, a projection apparatus 230, amotorized stage 209, and a sample holder 207. The electron source 201and projection apparatus 230 may together be referred to as anillumination apparatus. The sample holder 207 is supported by motorizedstage 209 so as to hold a sample 208 (e.g., a substrate or a mask) forinspection. Multi-beam electron beam tool 40 a further comprises anelectron detection device 1240. (Note this may be different in structurefrom the electron detection device 240 in the secondary electron-opticalcolumn of the embodiments referred to in FIGS. 2 and 3 , although it hasthe same function: to detect electrons from the sample).

Electron source 201 may comprise a cathode (not shown) and an extractoror anode (not shown). During operation, electron source 201 isconfigured to emit electrons as primary electrons from the cathode. Theprimary electrons are extracted or accelerated by the extractor and/orthe anode to form a primary electron beam 202.

Projection apparatus 230 is configured to convert primary electron beam202 into a plurality of sub-beams 211, 212, 213 and to direct eachsub-beam onto the sample 208. Although three sub-beams are illustratedfor simplicity, there may be many tens, many hundreds or many thousandsof sub-beams. The sub-beams may be referred to as beamlets.

Controller 50 of FIG. 1 may be connected to various parts of electronbeam tool 40 a such as electron source 201, electron detection device1240, projection apparatus 230, and motorized stage 209. Controller 50may perform various image and signal processing functions. Controller 50may also generate various control signals to govern operations of thecharged particle beam inspection apparatus, including the chargedparticle multi-beam apparatus.

Projection apparatus 230 may be configured to focus sub-beams 211, 212,and 213 onto a sample 208 for inspection and may form three probe spots221, 222, and 223 on the surface of sample 208. Projection apparatus 230may be configured to deflect primary sub-beams 211, 212, and 213 to scanprobe spots 221, 222, and 223 across individual scanning areas in asection of the surface of sample 208. In response to incidence ofprimary sub-beams 211, 212, and 213 on probe spots 221, 222, and 223 onsample 208, electrons are generated from the sample 208 which includesecondary electrons and backscattered electrons. The secondary electronstypically have electron energy ≤50 eV and backscattered electronstypically have electron energy between 50 eV and the landing energy ofprimary sub-beams 211, 212, and 213.

Electron detection device 1240 is configured to detect secondaryelectrons and/or backscattered electrons and to generate correspondingsignals which are sent to a controller or a signal processing system(not shown), e.g. to construct images of the corresponding scanned areasof sample 208. Electron detection device 1240 may comprise a detectormodule 402 integrated with an objective lens 401 as described above withreference to FIGS. 4 to 7 .

FIG. 11 is a schematic diagram illustrating an exemplary electron beamtool 40 b that may be part of the exemplary charged particle beaminspection apparatus 100 of FIG. 1 in place of the tool 40 of FIG. 2 .Parts of the apparatus 40 a that have similar functions as correspondingparts of the apparatus 40 of FIG. 2 are identified with the samereferences. A reduced or simplified description of such parts isincluded below in some cases.

Electron source 201 directs electrons toward an array of condenserlenses 1231 forming part of a projection system 230. The electron sourceis desirably a high brightness thermal field emitter with a goodcompromise between brightness and total emission current. There may bemany tens, many hundreds or many thousands of condenser lenses 1231.Condenser lenses 1231 may comprise multi-electrode lenses and have aconstruction based on European patent application publication no.EP1602121, which is hereby incorporated in its entirety by reference andin particular to the disclosure of a lens array to split an e-beam intoa plurality of sub-beams, with the array providing a lens for eachsub-beam. The array of condenser lenses may take the form of at leasttwo plates, acting as electrodes, with an aperture in each plate alignedwith each other and corresponding to the location of a sub-beam. Atleast two of the plates are maintained during operation at differentpotentials to achieve the desired lensing effect.

In an arrangement the array of condenser lenses is formed of three platearrays in which charged particles have the same energy as they enter andleave each lens, which arrangement may be referred to as an Einzel lens.The beam energy is the same on entering as leaving the Einzel lens.Thus, dispersion only occurs within the Einzel lens itself (betweenentry and exit electrodes of the lens), thereby limiting off-axischromatic aberrations. When the thickness of the condenser lenses islow, e.g. a few mm, such aberrations have a small or negligible effect.

The array of condenser lenses may comprise a plurality of beam apertures110. The beam apertures 110 may be formed, for example, by openings in asubstantially planar beam aperture body 111. The beam apertures 110divide a beam of charged particles from source 201 into a correspondingplurality of sub-beams. Each condenser lens in the array directselectrons into respective sub-beams 1211, 1212, 1213 which are focusedat respective intermediate focuses 1233. At the intermediate focuses1233 are deflectors 235. Deflectors 235 are configured to bend arespective sub-beam 1211, 1212, 1213 by an amount effective to ensurethat the principal ray (which may also be referred to as the beam axis)is incident on the sample 208 substantially normally (i.e. atsubstantially 90° to the nominal surface of the sample). Deflectors 235may also be referred to as collimators. In an embodiment, deflectors 235a can be provided additionally or alternatively. Downbeam (i.e. closerto the sample) of the intermediate focuses 1233 are a plurality ofobjective lenses 1234, each of which directs a respective sub-beam 1211,1212, 1213 onto the sample 208. Objective lenses 1234 can be configuredto demagnify the electron beam by a factor greater than 10, desirably inthe range of 50 to 100 or more.

An electron detection device 1240 is provided between the objectivelenses 1234 and the sample 208 to detect secondary and/or backscatteredelectrons emitted from the sample 208. Electron detection device 1240may comprise a detector module 402 integrated with an objective lens 401as described above with reference to FIGS. 4 to 7 . The electrondetection device 1240 may comprise sensor units for example captureelectrodes 402.

The system of FIG. 11 can be configured to control the landing energy ofthe electrons on the sample. The landing energy can be selected toincrease emission and detection of secondary electrons dependent on thenature of the sample being assessed. A controller provided to controlthe objective lenses 1234 may be configured to control the landingenergy to any desired value within a predetermined range or to a desiredone of a plurality of predetermined values. In an embodiment, thelanding energy can be controlled to desired value in the range of from1000 eV to 5000 eV. Details of electrode structures and potentials thatcan be used to control landing energy are disclosed in European patentapplication no. EPA 20158804.3, which is incorporated herein in itsentirety by reference.

In some embodiments, the charged particle assessment tool furthercomprises one or more aberration correctors that reduce one or moreaberrations in the sub-beams. In an embodiment, each of at least asubset of the aberration correctors is positioned in, or directlyadjacent to, a respective one of the intermediate foci (e.g. in oradjacent to the intermediate image plane). The sub-beams have a smallestsectional area in or near a focal plane such as the intermediate plane.This provides more space for aberration correctors than is availableelsewhere, i.e. upbeam (closer to the source) or downbeam (closer to thesample) of the intermediate plane (or than would be available inalternative arrangements that do not have an intermediate image plane).

In an embodiment, aberration correctors positioned in, or directlyadjacent to, the intermediate foci (or intermediate image plane)comprise deflectors to correct for the source 201 appearing to be atdifferent positions for different beams. Correctors can be used tocorrect macroscopic aberrations resulting from the source that prevent agood alignment between each sub-beam and a corresponding objective lens.

The aberration correctors may correct aberrations that prevent a propercolumn alignment. Such aberrations may also lead to a misalignmentbetween the sub-beams and the correctors. For this reason, it may bedesirable to, additionally or alternatively, have position aberrationcorrectors at or near the condenser lenses 1231 (e.g. with each suchaberration corrector being integrated with, or directly adjacent to, oneor more of the condenser lenses 1231). This is desirable because at ornear the condenser lenses 1231 aberrations will not yet have led to ashift of corresponding sub-beams because the condenser lenses 1231 arevertically close or coincident with the beam apertures. A challenge withpositioning correctors at or near the condenser lenses 1231, however, isthat the sub-beams each have relatively large sectional areas andrelatively small pitch at this location, relative to locations furtherdownbeam. The aberration correctors may be CMOS based individualprogrammable deflectors as disclosed in European patent applicationpublication no. EP2702595A1 or an array of multipole deflectors asdisclosed European patent application publication no. EP2715768A2, ofwhich the descriptions of the beamlet manipulators in both documents arehereby incorporated in their entirety by reference.

In some embodiments, each of at least a subset of the aberrationcorrectors is integrated with, or directly adjacent to, one or more ofthe objective lenses 1234. In an embodiment, these aberration correctorsreduce one or more of the following: field curvature; focus error;and/or astigmatism. Additionally or alternatively, one or more scanningdeflectors (not shown) may be integrated with, or directly adjacent to,one or more of the objective lenses 1234 for scanning the sub-beams1211, 1212, 1213 over the sample 208. In an embodiment, the scanningdeflectors described in U.S. patent application publication no. US2010/0276606, which is hereby incorporated in its entirety by reference,may be used.

In an embodiment the objective lens referred to in earlier embodimentsis an array objective lens. Each element in the array is a micro-lensoperating a different beam or group of beams in the multi-beam. Anelectrostatic array objective lens has at least two plates each with aplurality of holes or apertures. The position of each hole in a platecorresponds to the position of a corresponding hole in the other plate.The corresponding holes operate in use on the same beam or group ofbeams in the multi-beam. A suitable example of a type of lens for eachelement in the array is a two electrode decelerating lens.

An electron detection device 1240 is provided between the objectivelenses 1234 and the sample 208 to detect secondary and/or backscatteredelectrons emitted from the sample 208. Electron detection device maycomprise a detector module 402 integrated with an objective lens 401 asdescribed above with reference to FIGS. 4 to 7 . The electron detectiondevice 240 may comprise sensor units, for example capture electrodes405.

In an embodiment, the correctors 235 at the intermediate focuses 1233are embodied by a slit deflector. Slit deflector is an example of amanipulator and may also be referred to as a slit corrector.

An exemplary electron beam tool 40 c, that may be part of the exemplarycharged particle beam inspection apparatus 100 of FIG. 1 in place of thetool 40 of FIG. 2 , is illustrated schematically in FIG. 12 . Parts ofthe apparatus 40 a that have similar functions as corresponding parts ofthe apparatus 40 of FIG. 2 are identified with the same references. Areduced or simplified description of such parts is included below insome cases.

The tool 40 c further comprises one or more aberration correctors 124,125, 126 that reduce one or more aberrations in the sub-beams 114. In anembodiment, each of at least a subset of the aberration correctors 124is positioned in, or directly adjacent to, a respective one of theintermediate foci 115 (e.g. in or adjacent to the intermediate imageplane 120). The sub-beams 114 have a smallest cross-sectional area in ornear a focal plane such as the intermediate plane 120. This providesmore space for aberration correctors 124 than is available elsewhere,i.e. upbeam or downbeam of the intermediate plane 120 (or than would beavailable in alternative arrangements that do not have an intermediateimage plane 120).

In an embodiment, aberration correctors 124 positioned in, or directlyadjacent to, the intermediate foci 115 (or intermediate image plane 120)comprise deflectors to correct for the source 201 appearing to be atdifferent positions for different sub-beams 114 derived from beam 112emitted from source 201. Correctors 124 can be used to correctmacroscopic aberrations resulting from the source 201 that prevent agood alignment between each sub-beam 114 and a corresponding objectivelens 118.

The aberration correctors 124 may correct aberrations that prevent aproper column alignment. Such aberrations may also lead to amisalignment between the sub-beams 114 and the correctors 124. For thisreason, it may be desirable to, additionally or alternatively, positionaberration correctors 125 at or near the condenser lenses 116 (e.g. witheach such aberration corrector 125 being integrated with, or directlyadjacent to, one or more of the condenser lenses 116). This is desirablebecause at or near the condenser lenses 116 aberrations will not yethave led to a shift of corresponding sub-beams 114 because the condenserlenses 116 are vertically close or coincident with the beam apertures110. A challenge with positioning correctors 125 at or near thecondenser lenses 116, however, is that the sub-beams 114 each haverelatively large cross-sectional areas and relatively small pitch atthis location, relative to locations further downstream.

In some embodiments, as exemplified in FIG. 12 , each of at least asubset of the aberration correctors 126 is integrated with, or directlyadjacent to, one or more of the objective lenses 118. In an embodiment,these aberration correctors 126 reduce one or more of the following:field curvature; focus error; and/or astigmatism. In the apparatus ofFIG. 12 , any or all of the correctors 124, 125, 126 can be slitdeflectors.

FIGS. 13 and 14 depict an example of an electron detection device 240that can be used in an embodiment, for example it can be incorporated inthe electron beam tools 40, 40 a, 40 b, 40 c described above withreference for example to FIGS. 2, 10, 11 and 12 . FIG. 13 is a schematicside view of electron detection device 240 integrated in or associatedwith the objective lens array 501 and FIG. 14 is a view from belowelectron detection device 240.

As shown in FIG. 13 , electron detection device 240 in this examplecomprises a substrate 502 provided with a plurality of sensor units 503surrounding respective beam apertures 504. Substrate 502 is mounted tothe upper electrode (further from sample 208) of a decelerating arrayobjective lens 501. The sensor units 503 face toward the sample 208. Thesensor units may be positioned with the sensing surfaces located betweenthe upbeam and downbeam facing surfaces of the upper electrode. Thesensor units 503 may be integrated into or associated with the electrodeof the objective lens 501 furthest from the sample 208. This is incontrast to the electron detection device 240 of FIG. 7 which isintegrated into, or associated with, the lower electrode of an arrayobjective lens. That is in both embodiments the sensor units may beintegrated into the objective lens 501. (The sensor unit 503 of FIG. 7may be mounted to, but not necessarily integrated with, an electrode ofan array objective lens furthest from the source, or closest to thesample.) FIG. 13 depicts a two electrode objective lens but it will beappreciated that any other form of objective lens, e.g. a threeelectrode lens, may also be used.

The electron detection device 240 in this example is placed away fromthe electrode of the objective lens 501 furthest from the source, inother words away from an upbeam electrode of the objective lens 501. Inthis position, an electrode in the objective lens 501 is closer to thesample, or down-beam, of the electron detection device 240. Thus thesecondary electrons emitted by the sample 208 are accelerated by downbeam positioned electrode array of the objective lens 501, for exampleto many kV (perhaps about 28.5 kV). The substrate supporting the sensorunits 503 during operation may be held at the same potential differenceas the upper electrode. As a result the sensor units 503 can comprise,for example, PIN detectors and/or scintillators. This has an advantagethat no significant additional noise sources are present as PINdetectors and scintillators have a large initial amplification of thesignal. Another advantage of this arrangement is that it is easier toaccess the electron detection device 240, e.g. for making power andsignal connections or for servicing in use. Sensor units having captureelectrodes could be used at this location instead, but this could resultin poorer performance.

A PIN detector comprises a reverse-biased PIN diode and has an intrinsic(very lightly doped) semiconductor region sandwiched between a p-dopedand an n-doped region. Secondary electrons incident on the intrinsicsemiconductor region generate electron-hole pairs and allow a current toflow, generating a detection signal.

A scintillator comprises a material that emits light when electrons areincident on it. A detection signal is generated by imaging thescintillator with a camera or other imaging device.

In order to correctly image secondary electrodes on the sensor units 503it is desirable to provide a relatively large potential differencebetween last electrode and the sample 208. For example, the upperelectrode of the objective lens may be at about 30 kV, the lowerelectrode at about 3.5 kV and the sample 208 at about 2.5 kV. A largepotential difference between the lower electrode and the sample 208 mayincrease aberrations of the objective lens on the primary beam but asuitable trade-off can be selected.

Exact dimensions of an embodiment may be determined on a case-by-casebasis. The cross-sectional width (e.g., diameter) of the beam apertures504 may be in the range of about 5 to 20 μm, e.g. about 10 μm. The widthof the slits in the electrodes may be in the range of from 50 to 200 μm,e.g. about 100 μm. The pitch of the beam apertures and the electrodeslits may be in the range of from 100 to 200 μm, e.g. about 150 μm. Thegap between upper and lower electrodes may be in the range of from about1 and 1.5 mm, e.g. about 1.2 mm. The depth of the lower electrode may bein the range of from about 0.3 to 0.6 mm, e.g. about 0.48 mm. Theworking distance between the lower electrode and the sample 208 may bein the range of form about 0.2 to 0.5 mm, e.g. about 0.37 mm. Desirablythe electric field strength between the lower electrode and the sample208 is no more than about 2.7 kV/mm in order to avoid or reduce damageto the sample 208. The field in the gap between upper and lowerelectrodes can be larger, e.g. over 20 kV/mm.

The beam apertures 504 associated with the sensor units have smallercross-sectional widths (e.g., diameters) than the electrode arrays toincrease the surface of the sensor units available to capture electrodesemanating from the sample. However the dimensions of the beam aperturecross-sectional widths are selected so that they permit passage of thesub-beams; that is the beam apertures are not beam limiting. The beamapertures are design to permit passage of the sub-beams without shapingtheir cross-section. The same comment applies to the beam apertures 406associated with the sensor units 402 of the embodiment depicted in FIGS.4 to 7 .

In an embodiment, a single sensor unit (e.g. PIN detector) surroundseach aperture. The single sensor unit may have a circular perimeterand/or an outer diameter. The sensor unit may have an area extendingbetween the aperture and the periphery of the sensor unit. The sensorunits 503 may be arranged in rectangular array or a hexagonal array. Inan embodiment, a plurality of sensor elements (e.g. smaller PINdetectors) is provided around each aperture. The plurality of sensorelements may, together, have a circular perimeter and/or a diameter. Theplurality of sensor elements may, together, have an area extendingbetween the aperture and the periphery of the plurality of sensorelements. The pluralities of sensor elements may be arranged inrectangular array or a hexagonal array. The signal generated fromelectrons captured by sensor elements surrounding one aperture may becombined into a single signal or used to generate independent signals.The sensor elements may be divided radially. The sensor elements mayform a plurality of concentric annuluses or rings. The sensor elementsmay be divided angularly. The sensor elements may form a plurality ofsector-like pieces or segments. The segments may be of similar angularsize and/or similar area. The sensor elements may be divided bothradially and angularly or in any other convenient manner. The surfacesof the sensor units, optionally their sensor elements, may substantiallyfill the surface of the substrate supporting the sensor units.

FIG. 15 is a schematic diagram of an assessment tool. Parts that arecommon to earlier embodiments are indicated with the same referencenumeral and not described further below. Points of difference aredescribed.

Each condenser lens in the array 1231 directs electrons into arespective sub-beam 211, 212, 213 which are each focused at a respectiveintermediate focus 1233. Deflectors 235 are provided at the intermediatefocuses 1233.

Below (i.e. downbeam or further from source 201) deflectors 235 there isa control lens array 250 comprising a control lens for each sub-beam211, 212, 213. Control lens array 250 may comprise at least two, forexample three, plate electrode arrays connected to respective potentialsources. A function of control lens array 250 is to optimize the beamopening angle with respect to the demagnification of the beam and/or tocontrol the beam energy delivered to the objective lenses 234, each ofwhich directs a respective sub-beam 211, 212, 213 onto the sample 208.The control lenses pre-focus the sub-beams (e.g. apply a focusing actionto the sub-beams prior to the sub-beams reaching the objective lensarray). The pre-focusing may reduce divergence of the sub-beams orincrease a rate of convergence of the sub-beams. The control lens arrayand the objective lens array operate together to provide a combinedfocal length. Combined operation without an intermediate focus mayreduce the risk of aberrations. Note that the reference todemagnification and opening angle is intended to refer to variation ofthe same parameter. In an ideal arrangement the product ofdemagnification and the corresponding opening angle is constant over arange of values. However, the opening angle may be influenced by the useof an aperture. (It should be noted that in the arrangement shown inFIG. 15 adjustment of magnification results in similar adjustment of theopening angle because the beam current remains consistent along the beampath.)

The provision of a control lens array 250 in addition to an objectivelens array provides additional degrees of freedom for controllingproperties of the sub-beams as described in European patent applicationno. 20196716.3 filed 17 Sep. 2020, of which the portions referring touse and control of the control lens are hereby incorporated in theirentirety by reference. The additional freedom is provided even when thecontrol lens array 250 and objective lens array are provided relativelyclose together, for example such that no intermediate focus is formedbetween the control lens array 250 and the objective lens array. Ifthere are two electrodes then the demagnification and landing energy arecontrolled together. If there are three or more electrodes thedemagnification and landing energy can be controlled independently. Thecontrol lenses may thus be configured to adjust the demagnificationand/or beam opening angle of respective sub-beams (e.g. using theelectric power source to apply suitable respective potentials to theelectrodes of the control lenses and the objective lenses). Thisoptimization can be achieved with having an excessively negative impacton the number of objective lenses and without excessively deterioratingaberrations of the objective lenses (e.g. without increasing thestrength of the objective lenses).

Optionally an array of scan deflectors 260 is provided between thecontrol lens array 250 and the array of objective lenses 234. The arrayof scan deflectors 260 comprises a scan deflector for each sub-beam 211,212, 213. Each scan deflector is configured to deflect a respectivesub-beam 211, 212, 213 in one or two directions so as to scan the subbeam across the sample 208 in one or two directions.

An electron detection device 1240 is provided between the objectivelenses 234 and the sample 208 to detect secondary and/or backscatteredelectrons emitted from the sample 208. An exemplary construction of theelectron detection system is described below.

The system of FIG. 15 is configured to control the landing energy of theelectrons on the sample by varying the potentials applied to theelectrodes of the control lenses and the objective lenses. The controllenses and objective lenses work together and may be referred to as anobjective lens assembly. The landing energy can be selected to increaseemission and detection of secondary electrons dependent on the nature ofthe sample being assessed. A controller may be configured to control thelanding energy to any desired value within a predetermined range or to adesired one of a plurality of predetermined values. In an embodiment,the landing energy can be controlled to desired value in the range offrom 1000 eV to 5000 eV.

Desirably, the landing energy is primarily varied by controlling theenergy of the electrons exiting the control lens. The potentialdifferences within the objective lenses are desirably kept constantduring this variation so that the electric field within the objectivelens remains as high as possible. The potentials applied to the controllens in addition may be used to optimize the beam opening angle anddemagnification. The control lens can also be referred to as a refocuslens as it can function to correct the focus position in view of changesin the landing energy. The use of the control lens array enables theobjective lens array to be operated at its optimal electric fieldstrength.

In some embodiments, the charged particle assessment tool furthercomprises one or more aberration correctors that reduce one or moreaberrations in the sub-beams as discussed above.

In an embodiment, aberration correctors are positioned in, or directlyadjacent to, the intermediate foci (or intermediate image plane) asdescribed above.

In some embodiments, the detector 1240 of the objective lens assemblycomprises a detector array down-beam of at least one electrode of theobjective lens array. In an embodiment, the detector 1240 is adjacent toand/or integrated with the objective lens array. For example, thedetector array may be implemented by integrating a CMOS chip detectorinto a bottom electrode of the objective lens array.

In a variation on the embodiment of FIG. 15 , the condenser lens array1231 and collimators 235 are omitted, as disclosed in European patentapplication no. 20196714.8 filed on 17 Sep. 2020, which is incorporatedherein in its entirety by reference at least in so far as the disclosureof such electron-optical architecture. Such an arrangement may feature asource 201, a collimator (which may be a macro-collimator lens or acollimator lens array), a scan deflector (which may be a macro-scandeflector or a scan deflector array), a control lens, an objective lensarray and a detector array. The arrangement features a beam shapinglimiter (or beam shaping limiting array) and may feature an upper beamlimiter. The source 201 emits electrons toward an upper beam limiter,which defines an array of beam-limiting apertures. The upper beamlimiter may be referred to as an upper beam-limiting aperture array orup-beam beam-limiting aperture array. The upper beam limiter maycomprise a plate (which may be a plate-like body) having a plurality ofapertures. The upper beam limiter forms sub-beams from a beam of chargedparticles emitted by the source 201. The upper beam limiter may beassociated with the control lens array and may form the most-upbeamelectrode of the control lens array. Portions of the beam other thanthose contributing to forming the sub-beams may be blocked (e.g.absorbed) by the upper beam limiter, for example, so as not to interferewith the sub-beams down-beam. The collimator array (e.g. formed usingMEMS manufacturing techniques) collimates the individual sub-beams, andmay direct the sub-beams to the control lenses. In this variation,optionally the upper beam limiter, collimator element array, controllenses 250, scan deflector array 260, objective lenses 234, beam shapinglimiter and detector module 1240 can all be formed using MEMSmanufacturing techniques.

The beam shaping limiter is associated with the objective lenses andshapes the sub-beams down-beam of the control lenses. The scan deflectorscans the sub-beams defined upbeam of the beam-shaping limiter over thebeam shaping limiter. The beam shaping limiter shapes the sub-beams thatare incidental on the sample surface. Use of the beam shaping limitermay reduce, if not minimize, the aberrations contributed by the controllenses. As the beam shaping limiter is down beam of the control lensarray, the apertures in the beam shaping limiter adjust the beam currentalong the beam path. Thus control of the magnification by the controllens operates differently on the opening angle. That is the apertures inthe beam shaping limiter break the direct correspondence betweenvariations in the magnification and opening angle

In a tool having a variable landing energy, such as that described abovewith reference to FIG. 15 , the Z position (i.e. position along the beampath) of the focus changes with landing energy. The main reason for thisis that the focal length of the objective lens is roughly equal to 4times the landing energy divided by the electrostatic field in theobjective lens. In order to improve the aberration level of theobjective lens it is desired to keep the electrostatic field as high aspossible. As a result the focal length scales proportional to thelanding energy. It is possible to reduce the electrostatic field in theobjective lens if the Z position of the focus is too close to theobjective lens but this results in a loss of resolution. Conventionally,the sample is moved in the Z direction to make sure the primary beamsare correctly focused on the substrate. In one arrangement, the Zposition of the focus can vary up to 1 mm for a change of landing energybetween 500V and 5 kV and as a result the measurement signal, which isdependent on distance between sample and detector, varies substantially.The relationship between change in landing energy and change in Zposition of the focus depends in part on the lens strength of theobjective lens so in other arrangements the range of variation in the Zposition of the focus may be greater or less than 1 mm. There may be alinear relationship between landing energy and focal distance. In theabove range of landing energies, the resolution can substantively bemaintained.

According to an embodiment it is proposed to maintain the position ofthe detector relative to the sample, even if the sample is movedrelative to the objective lens due to a change in focus position, e.g.as a result of a change in landing energy. For example, the distancebetween sample and detector is maintained in the range of about 50 to100 μm. In an embodiment, the distance between the objective lens andthe sample may be about 250 μm or more. However there is a lower limitto the proximity in which the objective lens may be positioned relativeto a sample and thus the proximity of the focus of the sub-beams to theobjective lens. In such a situation there is a risk that the objectivelens electrode might need to be too thin to be easily manufacturable. Adetector used with such an arrangement might need to be too thin to beeasily manufacturable. The desirable distance between sample anddetector may depend on the detector size (specifically the electrodecross-sectional width) and/or the detector pitch. All other things beingequal, a larger detector and/or a larger detector pitch may allow alarger distance between sample and detector. An embodiment can thereforemaintain a high secondary electron detection at a given beam pitch anddetector cross-sectional width.

Two approaches to maintaining essentially constant the distance betweensample and detector are proposed. As depicted in FIG. 16 , the detectormodule 240 is connected to an actuator system 245 which is configured toposition the detector module 245 in the direction parallel to thedirection of propagation of the electron beams, i.e. perpendicular tothe surface of the sample. Within FIGS. 16 , A, B and C show thearrangement with the detector in different vertical positions. Theactuation system 245 can be connected to the overall control system 50so as to maintain the detection module 245 at a substantially constantdistance from the sample. That is even as the sample is moved toposition its surface at a focal position that changes, e.g. due tochanges in the landing energy of the electron beams. It may not benecessary to maintain the distance between detector module 240 andsample 208 exactly constant. Rather it can be sufficient to reduce thevariation in distance to an acceptable level. Actuation system 245 mayinclude various different types of actuators, e.g. one or morepiezo-electric actuators and/or one or more Lorentz actuators. Oneactuator may be sufficient to position all sensor units of a detectormodule or multiple actuators may be used, each positioning a group ofsensor units. It is also possible to have one or more actuators persensor unit. As the detectors may be in an array on a substrate theactuator arrangement may actuate the substrate. Desirably an actuator isable to reposition the detector module in a matter of seconds or less.

As well as positioning the detector in Z, the actuator system 245 may beconfigured to position the detector in other degrees of freedom, such asRx and/or Ry. However providing actuation in additional degrees offreedom may undesirably increase complexity.

In another approach, the detector is exchangeable. In an embodimentdepicted in FIG. 17 , two or more, e.g. three or four or five, detectormodules are interchangeable. Each interchangeable detector module 240 a,240 b, 240 c, is configured to have the charged-particle receivingsurfaces of its sensor units at a different vertical position relativeto objective lenses 401. For example, each detector module 240 a, 240 b,240 c may be formed on a substrate of different dimensions, e.g.thickness. Alternatively or in addition, spacers of differentthicknesses may be provided. Such a spacer may be used to space thedetector relative to the objective lens array. Detector modules may beinterchangeable on their own or in combination with other elements suchas the objective lens assembly, the objective lens array, beam shapinglimiter, upper limiting array, collimator array, scan deflector arrayand/or control lens array. The different electron-optical components mayhave their own designated modules. They may be comprised with otherelectron-optical components in the same module so that there are fewermodules than the number of exchangeable electron-optical component.Alternatively all exchangeable electron-optical components, desirablythe MEMS elements, for example, of the objective lens assembly may be inan exchangeable module. In an arrangement a module may comprise anactuator, for example, to actuate the detector array relative to otherelectron-optical components in the module. Spacers used to space theelectron-optical components may be exchangeable. Spacers may beincorporated in a module with multiple electron components, between theelectron-optical components.

Desirably, an automated interchange mechanism is provided so that amodule, such as the detector module, can be swapped between operable andnon-operable positions without opening the tool, e.g. in betweenassessments of successive samples or batches of samples. Alternatively,a module, such as the detector module, may be manually interchangeable,e.g. field replaceable. A field replaceable module may be removed andreplaced with the same or different module while maintaining the vacuumin which the electron-optical tool 40 is located as described in U.S.patent application No. 63/037,481 filed on 10 Jun. 2020, which isincorporated in its entirety by reference at least in so far as thefeatures enabling a replaceable module is hereby incorporated byreference. Only a section of the column corresponding to the module tobe replaced is vented for the module to be removed and returned orreplaced. Although this is less desirable than an automated interchangemechanism since opening the tool increases downtime, it may still bebeneficial where long runs of measurements at the same beam settings areto be performed. If an automated interchange device is provided theexchange of modules may take of the order of minutes whereas a manualexchange may take of the order of hours. Actuating an electron-opticalcomponent, such as the detector, may be quicker than either an automatedinterchange of modules or a manual exchange, taking a matter of seconds.The vertical position of the detector module may also be controlled byuse of an interchangeable spacer which may be interchangeable by anautomated or manually operated arrangement as described with respect tothe electron-optical module. The interchangeable spacer may beincorporated in an exchangeable module.

An assessment tool according to an embodiment of the invention may be atool which makes a qualitative assessment of a sample (e.g. pass/fail),one which makes a quantitative measurement (e.g. the size of a feature)of a sample or which generates an image of map of a sample. Examples ofassessment tools are inspections tools and metrology tools.

The following clauses are exemplary embodiments of the invention:

Clause 1: A charged particle assessment tool comprising: an objectivelens configured to project a plurality of charged particle beams onto asample, the objective lens having a sample-facing surface defining aplurality of beam apertures through which respective ones of the chargedparticle beams are emitted toward the sample; and a plurality of captureelectrodes adjacent respective ones of the beam apertures and configuredto capture charged particles emitted from the sample.

Clause 2: A tool according to clause 1, wherein each capture electrodeis configured to substantially surround a respective beam aperture.

Clause 3: A tool according to clause 1 or clause 2, wherein the captureelectrodes are configured to substantially fill the sample-facingsurface.

Clause 4: A tool according to clause 1 or clause 2, wherein the captureelectrodes have a circular outer perimeter.

Clause 5: A tool according to any of clauses 1 to 4, further comprisinga substrate mounted on the sample-facing surface of the objective lensand on which the capture electrodes are formed.

Clause 6: A tool according to clause 5, further comprising controlcircuitry formed in the substrate.

Clause 7: A tool according to clause 6, wherein the control circuitrycomprises one or more selected from: an amplifier, e.g. a transimpedanceamplifier; an analog-to-digital converter; a data multiplexor; and/orread-out gates.

Clause 8: A tool according to clause 7, wherein the control circuitrycomprises one amplifier for each capture electrode.

Clause 9: A tool according to any of clauses 5 to 8, further comprisingconductive traces provided on the other side of the substrate to thecapture electrodes.

Clause 10: A tool according to any of clauses 5 to 9, further comprisingvias passing though the substrate.

Clause 11: A tool according to any of clauses 5 to 10, wherein thesubstrate is formed of silicon.

Clause 12: A tool according to any of clauses 1 to 11, wherein thecapture electrodes are formed by a CMOS process.

Clause 13: A tool according to any of clauses 1 to 12, wherein eachcapture electrode comprises a plurality of electrode elements.

Clause 14: A method of manufacturing an assessment tool, the methodcomprising: forming a plurality of capture electrodes on, and aplurality of apertures in, a substrate; and attaching the substrate toan objective lens configured to project a plurality of charged particlebeams onto a sample, so that the charged particle beams can be emittedthrough the apertures.

Clause 15: A method according to clause 14, wherein the apertures areformed by etching through the substrate.

Clause 16: An inspection method comprising: emitting a plurality ofcharged-particle beams through a plurality of beam apertures to asample; and capturing charged particles emitted by the sample inresponse to the charged-particle beams using a plurality of captureelectrodes provided adjacent respective ones of the beam apertures.

Clause 17: A multi-beam electron-optical system comprising a lastelectron-optical element in a multi-beam path of the multi-beam electronoptical system, the last electron-optical element comprising: amulti-manipulator array in which each array element is configured tomanipulate at least one electron beam in the multi-beam path; and adetector configured and orientated to detect electrons emitted from asample positioned in the multi-beam beam path, wherein the detectorcomprises a plurality of electrodes integrated into themulti-manipulator array and at least one electrode associated each arrayelement.

Clause 18: A last electron-optical element for a multi-charged beamprojection system configured to project a plurality of charged particlebeams onto a sample, the last electron-optical element comprising: anobjective lens having an sample-facing surface defining a plurality ofbeam apertures through which respective ones of the charged particlebeams are emitted toward the sample; and a plurality of captureelectrodes adjacent respective ones of the beam apertures and configuredto capture charged particles emitted from the sample.

Clause 19: A charged particle assessment tool comprising: an objectivelens configured to project a plurality of charged particle beams onto asample, the objective lens defining a plurality of beam aperturesthrough which respective ones of the charged particle beams canpropagate toward the sample; and a plurality of sensor units adjacentrespective ones of the beam apertures and configured to capture chargedparticles emitted from the sample.

Clause 20: A tool according to clause 19, wherein each sensor unit isconfigured to substantially surround a respective beam aperture.

Clause 21: A tool according to clause 19 or clause 20, wherein thesensor units have a circular outer perimeter.

Clause 22: A tool according to any of clauses 19 to 21, furthercomprising a substrate provided at a downbeam-facing surface of theobjective lens and on which the sensor units are formed.

Clause 23: A tool according to clause 22, wherein the sensor units areconfigured to substantially fill the sample-facing surface.

Clause 24: A tool according to clause 22 or clause 23, wherein thesensor units are capture electrodes.

Clause 25: A tool according to any of clauses 19 to 24, furthercomprising a substrate provided at an upbeam-facing surface of theobjective lens and on which the sensor units are formed, preferably thesensor units are configured to face downbeam.

Clause 26: A tool according to clause 25, wherein the sensor units areselected from the group consisting of PIN detectors and scintillators.

Clause 27: A tool according to any of clauses 22 to 26, furthercomprising control circuitry formed in the substrate.

Clause 28: A tool according to clause 27, wherein the control circuitrycomprises one or more selected from: an amplifier, e.g. a transimpedanceamplifier; an analog-to-digital converter; a data multiplexor; and/orread-out gates.

Clause 29: A tool according to clause 28, wherein the control circuitrycomprises one amplifier for each sensor unit.

Clause 30: A tool according to any of clauses 22 to 29, furthercomprising conductive traces provided on the other side of the substrateto the sensor unit.

Clause 31: A tool according to any of clauses 22 to 30, furthercomprising vias passing though the substrate.

Clause 32: A tool according to any of clauses 22 to 31, wherein thesubstrate is formed of silicon.

Clause 33: A tool according to any of clauses 19 to 32, wherein thesensor units are formed by a CMOS process.

Clause 34: A tool according to any of clauses 19 to 33, wherein eachsensor unit comprises a plurality of sensor elements.

Clause 35: A tool according to any of clauses 19 to 34, wherein theobjective lens is an electrostatic lens.

Clause 36: A tool according to any of clauses 19 to 35, furthercomprising an actuation system configured to adjust the position of thesensor units in a direction parallel to the direction of propagation ofthe electron beams.

Clause 37: A tool according to any of clauses 19 to 36 comprising: afirst sensor unit array; a second sensor unit array; and an interchangemechanism configured to selectively position either of the first andsecond sensor unit arrays at a downbeam-facing surface of the objectivelens, wherein the first and second sensor unit arrays are configuredsuch that, when the respective array is positioned at thedownbeam-facing surface, the sensor units of the first sensor unit arrayare positioned at a different distance from the objective than are thesensor units of the second sensor unit array.

Clause 38: A charged particle assessment tool comprising: an objectivelens configured to project a plurality of charged particle beams onto asample through a plurality of beam apertures defined in the objectivelens; and a sensor array comprising sensor units adjacent respectivebeam apertures and configured to capture charged particles emitted fromthe sample, wherein the sensor array is configured to be adjustablebetween positions along beam paths of the charged particle beams.

Clause 39: A tool according to clause 38, wherein the sensor array isconfigured to be adjustable by actuating the sensor array along the beampaths.

Clause 40: A tool according to clause 39, further comprising an actuatorconfigured to actuate the sensor array along the beam paths.

Clause 41: A charged particle assessment tool comprising: an objectivelens configured to project a plurality of charged particle beams onto asample through a plurality of beam apertures defined in the objectivelens; and a sensor array comprising sensor units adjacent respectivebeam apertures and configured to capture charged particles emitted fromthe sample, wherein the sensor array is configured to be actuatablealong beam paths of the charged particle beams.

Clause 42: A tool according to any of clauses 36 to 41, furthercomprising a beam energy control system configured to control thelanding energy of the electron beams on the sample.

Clause 43: A tool according to any of clauses 36 to 42, furthercomprising a control lens array upbeam of the objective lens array.

Clause 44: A tool according to clause 43, wherein the objective lensarray and control lens array comprises at least electrodes configured inoperation such that the objective lens focuses the charged particle beamonto the sample and the control lens adjusts the beam opening angleand/or demagnification.

Clause 45: A multi-beam charged particle optical column configured todirect a multi-beam towards a sample, the multi-beam being generateddown-beam from a source, the column comprising: a detector configured tocapture charged particles emitted from the sample, wherein the detectoris actuatable along the beam path.

Clause 46: A multi-beam charged particle optical column according toclause 45 wherein the detector comprises a sensor array, each sensorassigned to a respective sub-beam of the multi-beam.

Clause 47: A multi-beam charged particle optical column according toclause 45 or clause 46, wherein the column comprises a beam limitingaperture array configured to generate the multi-beams derived from asource beam.

Clause 48: A multi-beam charged particle optical column according toclause 47, wherein the detector is down-beam of the beam-limitingaperture array.

Clause 49: A multi-beam charged particle optical column according to anyof clauses 45 to 48, wherein the detector is integrated into theobjective lens assembly comprising an objective lens.

Clause 50: A method of manufacturing an assessment tool, the methodcomprising: forming a plurality of sensor units on, and a plurality ofapertures in, a substrate; and attaching the substrate to an objectivelens configured to project a plurality of charged particle beams onto asample, so that the charged particle beams can be emitted through theapertures.

Clause 51: A method according to clause 50, wherein the apertures areformed by etching through the substrate.

Clause 52: An inspection method comprising: emitting a plurality ofcharged-particle beams through a plurality of beam apertures to asample; and capturing charged particles emitted by the sample inresponse to the charged-particle beams using a plurality of sensor unitsprovided adjacent respective ones of the beam apertures.

Clause 53: A method according to clause 52, further comprising changingthe position of the sensor units along a path of the charged particlebeams.

Clause 54: A multi-beam electron-optical system comprising a lastelectron-optical element in a multi-beam path of the multi-beam electronoptical system, the last electron-optical element comprising: amulti-manipulator array in which each array element is configured tomanipulate at least one electron beam in the multi-beam path; and adetector configured and orientated to detect electrons emitted from asample positioned in the multi-beam beam path, wherein the detectorcomprises a plurality of sensor units integrated into themulti-manipulator array and at least one sensor unit associated eacharray element.

Clause 55: A last electron-optical element for a multi-charged beamprojection system configured to project a plurality of charged particlebeams onto a sample, the last electron-optical element comprising: anobjective lens having an sample-facing surface defining a plurality ofbeam apertures through which respective ones of the charged particlebeams can propagate toward the sample; and a plurality of sensor unitsadjacent respective ones of the beam apertures and configured to capturecharged particles emitted from the sample.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made as described without departing from the scope of the claimsset out below.

The invention claimed is:
 1. A multi-beam charged particle opticalcolumn configured to direct a multi-beam towards a sample, themulti-beam being generated down-beam from a source, the columncomprising: an objective lens configured to project a plurality ofcharged particle beams of the multi-beam onto the sample, the objectivelens defining a plurality of beam apertures through which respectiveones of the charged particle beams can propagate toward the sample; anda detector configured to capture charged particles emitted from thesample, wherein the detector comprises sensor units adjacent the beamapertures and configured to capture charged particles emitted from thesample and wherein the detector is actuatable along the path of themulti-beam.
 2. The optical column of claim 1, comprising a beam limitingaperture array configured to generate the multi-beams derived from asource beam.
 3. The optical column of claim 2, wherein the detector isdown-beam of the beam-limiting aperture array.
 4. The optical column ofclaim 1, wherein each sensor unit is configured to substantiallysurround a respective beam aperture.
 5. The optical column of claim 1,wherein the sensor units have a circular outer perimeter.
 6. The opticalcolumn of claim 1, wherein the sensor units are integrated into anobjective lens assembly comprising the objective lens.
 7. The opticalcolumn of claim 6, further comprising a substrate provided at adownbeam-facing surface of the objective lens and on which the sensorunits are formed.
 8. The optical column according to claim 7, whereinthe sensor units are configured to substantially fill the sample-facingsurface.
 9. The optical column according to claim 7, wherein the sensorunits are capture electrodes.
 10. The optical column according to claim7, further comprising control circuitry formed in the substrate.
 11. Theoptical column according to claim 7, further comprising vias passingthough the substrate.
 12. The optical column according to claim 6,wherein the objective lens assembly comprises a beam energy controlsystem configured to control landing energy of the electron beams on thesample.
 13. The optical column according to claim 6, wherein theobjective lens assembly comprises a control lens array upbeam of theobjective lens array.
 14. The optical column according to claim 1,further comprising a substrate provided at an upbeam-facing surface ofthe objective lens and on which the sensor units are formed.
 15. Acharged particle assessment tool comprising: the optical columnaccording to claim 1; and an actuator configured to actuate the detectoralong the path of the multi-beam.
 16. An inspection method comprising:emitting a multi-beam of a plurality of charged-particle beams through aplurality of beam apertures to a sample; and capturing charged particlesemitted by the sample in response to the charged-particle beams using adetector, wherein the detector comprises sensor units adjacentrespective beam apertures and capturing the charged particles emittedfrom the sample; and changing a position of the detector along a path ofthe multi-beam.
 17. A charged particle assessment tool comprising: anobjective lens configured to project a plurality of charged particlebeams onto a sample through a plurality of beam apertures defined in theobjective lens; and a sensor array comprising sensor units adjacentrespective beam apertures and configured to capture charged particlesemitted from the sample, wherein the sensor array is configured to beadjustable between positions along beam paths of the charged particlebeams.
 18. The tool according to claim 17, wherein the sensor array isconfigured to be adjustable by actuating the sensor array along the beampaths.
 19. The tool according to claim 18, further comprising anactuator configured to actuate the sensor array along the beam paths.20. The tool according to claim 17, wherein each sensor unit isconfigured to substantially surround a respective beam aperture.